Power plants – Internal combustion engine with treatment or handling of... – Having sensor or indicator of malfunction – unsafeness – or...
Reexamination Certificate
2000-10-11
2003-10-07
Denion, Thomas E. (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Having sensor or indicator of malfunction, unsafeness, or...
C060S285000, C060S286000, C060S301000
Reexamination Certificate
active
06629408
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to an exhaust emission control system including a NOx removing device for removing NOx (nitrogen oxides) and having a function of determining deterioration of the NOx removing device.
When the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set in a lean region with respect to a stoichiometric ratio (i.e., in the case of carrying out a so-called lean operation), the emission amount of NOx tends to increase. To cope with this, a known technique for exhaust emission control includes providing a NOx removing device containing a NOx absorbent for absorbing NOx in the exhaust system of the engine. The NOx absorbent has a characteristic that when the air-fuel ratio is set in a lean region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively high (the amount of NOx is large) (this condition will be hereinafter referred to as “exhaust lean condition”), the NOx absorbent absorbs NOx. When the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively low (this condition will be hereinafter referred to as “exhaust rich condition”), the NOx absorbent discharges the absorbed NOx. The NOx removing device containing this NOx absorbent is configured so that NOx discharged from the NOx absorbent in the exhaust rich condition is reduced by HC and CO and then exhausted as nitrogen gas, while HC and CO are oxidized by NOx and then exhausted as water vapor and carbon dioxide.
There is naturally a limit to the amount of NOx that can be absorbed by the NOx absorbent, and this limit tends to decrease with deterioration of the NOx absorbent. A technique of determining a degree of deterioration of the NOx absorbent is known in the art (Japanese Patent Laid-open No. Hei 10-299460). In this technique, two oxygen concentration sensors are arranged upstream and downstream of the NOx removing device, and air-fuel ratio enrichment for discharging the NOx absorbed by the NOx absorbent is carried out. Then, the degree of deterioration of the NOx absorbent is determined according to a delay time period from the time when an output value from the upstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio to the time when an output value from the downstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio.
However, the above delay time period changes with engine operating condition, (engine rotational speed and engine load), so that unless the engine operating condition for the execution of deterioration determination is limited to a narrow range, an improper determination is likely. Changing the deterioration determination threshold according to the engine operating condition is one method for solving this problem. However, improper determination is prone to occur in transient conditions where the engine operating condition changes.
Further, the NOx absorbing capacity of the NOx absorbent is reduced not only by the deterioration of the NOx absorbent, but also by absorption of sulfur oxides (SOx) contained in fuel components. This absorption is called sulfur poisoning. In the conventional deterioration determining technique, however, sulfur poisoning is not taken into consideration, Therefore, there is a possibility that it may be improperly determined that the NOx absorbent itself is deteriorated when the NOx absorbing capacity is reduced by sulfur poisoning. The NOx absorbing capacity reduced by sulfur poisoning can be recovered by regeneration processing. Accordingly, it is undesirable to determine that the NOx absorbent is in an unrecoverable deteriorated condition in the case of sulfur poisoning.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an exhaust emission control system which can perform accurate determination of deterioration of a NOx removing device regardless of an engine operating condition.
It is another object of the present invention to provide an exhaust emission control system which can accurately determine deterioration of a NOx removing device and has a function of reliably regenerating the NOx removing device in the case that it is regenerable.
In accordance with an aspect of the present invention, there is provided an exhaust emission control system for an exhaust system for an internal combustion engine, having a nitrogen oxide removing means for absorbing NOx contained in exhaust gases in an exhaust lean condition. This system further comprises the following: first and second oxygen concentration sensors respectively provided upstream and downstream of the nitrogen oxide removing means for detecting the oxygen concentration in the exhaust gases; deterioration-determination enriching means for changing the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to a stoichiometric ratio; reducing-component amount calculating means for calculating an amount of reducing components flowing into the nitrogen oxide removing means from the time when an output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio after the enrichment executed by the deterioration determination enriching means; and deterioration determining means for determining deterioration of the nitrogen oxide removing means according to the amount of reducing components calculated by the reducing-component amount calculating means and an output value from the second oxygen concentration sensor.
With this arrangement, after changing the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to the stoichiometric ratio, an amount of reducing components flowing into the nitrogen oxide removing means is calculated from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio, and the deterioration of the nitrogen oxide removing means is determined according to the calculated amount of reducing components and the output value from the second oxygen concentration sensor. Accordingly, the deterioration determination can be performed according to the amount of reducing components changing with an engine operating condition to thereby make it possible to perform accurate determination of deterioration of the nitrogen oxide removing means in a wide range of engine operating conditions.
Preferably, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the output value from the second oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio before the amount of reducing components reaches a predetermined amount.
As a modification, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the amount of reducing components at the time the output value from the second oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio is smaller than a predetermined amount.
As another modification, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the output value from the second oxygen concentration sensor indicates a rich air-fuel ratio at the time the amount of reducing components has reached a predetermined amount.
Preferably, the reducing-component amount calculating means calculates the amount of reducing components by integrating an amount of exhaust gases flowing into the nitrogen oxide removing means.
With this configuration, the amount of reducing components is calculated by integrating the amount of exhaust gases flowing into the nitrogen oxide removing means, so that the calculation of the reducing component amount can be easily
Hashimoto Akira
Hashizume Mistuo
Miyashita Kotaro
Murakami Akira
Takanohashi Toshikatsu
Arent Fox Kintner Plotkin & Kahn
Denion Thomas E.
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